Many people suffer from myopia (short-sightedness). The prevalence of myopia is increasing, leading to increased attention to the development of solutions. In addition, for many people, myopia progresses over time, despite correction using some existing methods.
FIG. 1 shows an eye that is normal sighted (i.e. is neither long-sighted nor short-sighted). FIG. 2 shows a myopic eye viewing a distant object; the focal point of the image is located in front of the retina. This shifted focal point relative to the retina creates blur.
Several techniques have been used to correct myopia. These techniques include prescribing spectacle lenses or contact lenses or intra-ocular lenses, surgical reshaping of the cornea and temporary reshaping of the cornea by hard or soft contact lenses.
When looking at near objects, it has been observed that many individuals with myopia accommodate less than that which is needed to bring the image forward onto the retina. This under-accommodation is often referred to as a lag of accommodation. FIG. 3 shows a myopic eye with a lag of accommodation; the focal point of the image is located behind the retina. Studies involving children indicate that lag of accommodation typically increases with increasing near focus (i.e. accommodation) demand. In a study involving children of primarily European descent, lag of accommodation measured at 33 centimetres using an autorefractor found the median lag to be 1.26 D (range from −0.75 to 2.82 D) in children aged 8 to 11 yrs. In children of Chinese ethnicity, lag of accommodation measured at 33 centimetres was 0.74+1-0.27 D.
Patent publication EP 2004/005560 A1 to Radhakrishnan et al describes a method said to retard or control the progression of myopia by controlling aberrations, thereby manipulating the position of the medium and high spatial frequency peaks of a visual image in a predetermined fashion. The repositioning of medium and high spatial frequency peaks intending to alter accommodative lag. The method calls for providing an ocular system of a predetermined aberration controlled design and for the design to provide negative spherical aberration.
International patent publication WO 05/055891 A1 describes the use of a contact lens to control the relative curvature of field with an objective of controlling the progression of myopia. The method includes moving peripheral images forwards relative to the retina, while allowing clear central vision.
U.S. Pat. No. 6,752,499 (Aller) describes the use of multifocal contact lenses to control the progression of myopia in myopic eyes with esofixation disparity. Aller describes providing a lens that provides for acceptable distance visual acuity and reduces or corrects esophoria at near. Aller describes use of near centre bifocal lenses having an add power of up to 2.25 D and the use of distance centre lenses with add powers of up to 2.5 D.
Multifocal and bifocal contact lenses have also been designed for presbyopic eyes.
U.S. Pat. No. 6,457,826 (Lett) describes a centre near bifocal lens and a centre distance bifocal lens. A described embodiment of a centre near bifocal lens has a constant power centre area extending to a chord diameter of 2.07 mm, a distance power outer area commencing at a chord diameter of 2.71 mm and a gradient power aspheric area that provides a continuous power transition from the centre area to the outer area. For a 3.0 mm pupil, Lett says that the near power occupies 48% of the pupil area and the distance power 18%. For a 5.0 mm pupil Lett says that the near power occupies 17% of the pupil and the distance power 71%.
U.S. Pat. No. 5,139,325 (Oksman et al) describes a lens with a vision correction power that is inversely proportional to the radial distance of the lens. In a described example, a lens has an add power over distance vision of 2.75 diopters centrally up to a radius of 0.72 mm, with the add power decreasing inversely proportional with radius after 0.72 mm. Another example has an add power over distance vision of 3.00 diopters up to a radius of 0.66 mm. The add power is described as not reaching zero unless the function is truncated.
U.S. Pat. No. 5,754,270 (Rehse et al) describes a lens with a central aspheric zone with an add power over distance vision of between 2.25 to 2.50 D up to a diameter of about 2.4 mm, a change in add power of about 0.5 to 1.25 D over the area between the diameters of 2.4 mm and 2.5 mm and then a progressive reduction in add power down to the power required for distance vision correction at about 6 mm diameter.
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.